• Lagostomus;
  • pituitary gland;
  • lactotrophs;
  • reproductive cycle;
  • melatonin;
  • age;
  • sex


Lactotrophs in pituitary pars distalis (PD) of viscacha were studied by immunohistochemistry and morphometric analysis in the following groups: 1) adult males throughout the reproductive cycle (reproductive, gonadal regression, and recovery periods), 2) melatonin-treated adults, 3) castrated adults, 4) prepubertal, 5) non-pregnant females, and 6) pregnant females (early, mid, and late pregnancy). Immunopositive percentage area (%IA), cell percentage in PD (% PDC), number of cells per reference area (no.cell/RA), major cellular and nuclear diameters were analyzed. Lactotrophs were mainly localized in the ventro–medial region and the caudal extreme of PD. In the male viscachas, they were isolated in small and big groups, close to blood vessels and near follicles. These cells were pleomorphic and with a heterogeneous cytoplasmic immunolabeling pattern. In the adult males of the gonadal regression period the morphometric parameters were the lowest. Most parameters of lactotrophs in the prepubertal were significantly lower than in the adult males in the reproductive period. In the melatonin-treated animals and in castrated animals there was a decrease in %IA, %PDC, and no.cell/RA. In the females, the morphometric parameters increased at the end of pregnancy. Non-pregnant females exhibited a higher immunopositive area and number, but a smaller size of cells than males. Our results showed that in the adult male viscacha, lactotrophs vary seasonally, probably due to the photoperiod effect through melatonin. Besides the changes observed after castration, in prepubertal animals, in adults of different sex, and during pregnancy suggest that the gonadal steroid hormones might modify the lactotrophs activity. Anat Rec, 293:150–161, 2010. © 2010 Wiley-Liss, Inc.